If your cleats shift, you're going to notice. You can reposition them (especially when the cleats are new), and it's usually obvious where they should be if you look at the sole -- there's an imprint. If they're not well positioned, it can hurt, and you need to get it fixed. Fortunately, that's likely to happen during your training rides so you can fix it before you leave on your trip. (You are doing or gonna do some training, right?)

I'm something of an outlier; I couldn't find a comfortable position for SPD cleats. I switched to Speedplay Frog pedals, and have been riding them happily for about 18 years -- including a cross-country ride and several shorter rides. My reasoning goes something like this: If I pedal too slowly on climbs, my knees tell me about it. So I try to keep my cadence up. I had some experiences with landing on the top tube when I was younger, and don't want to repeat that. Clipless keeps my feet attached to the pedals so I don't slip off. Clips and straps would work, too, but for riding in town or city traffic (which happens with a certain regularity while touring), clipless works better for me than a cinched-down strap.

Ride what you want, have a great trip, have fun!